CN114755310B - Method for predicting evolution rule of fractured reservoir rock mechanical layer - Google Patents
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Abstract
The invention relates to the field of oil and gas field exploration and development, in particular to a method for predicting the evolution rule of a rock mechanical layer of a fractured reservoir. Establishing a three-dimensional heterogeneous model of the mechanical parameters of the complete rock through core experiments, logging calculation and seismic inversion; establishing a three-dimensional fracture discrete network geomechanical model through field observation; analyzing the influence of the fracture parameters on the mechanical parameters and the anisotropism of the fractured rock mass through numerical simulation; and combining the relation between the fracture parameters of the reservoir and the rock mechanical parameters and stress fields, sequentially and circularly simulating the ancient stress fields in different periods and the densities and the shapes of the cracks in different periods, and interpreting the migration rule of the rock mechanical layer under the control of the construction factors. The invention provides a method for predicting the evolution law of a mechanical layer of a fractured reservoir rock from the angles of physical simulation and numerical simulation, and has certain reference significance for the aspects of predicting the evolution law of the mechanical property of the fractured reservoir rock, analyzing the mechanism of the fracture origin and the like.
Description
Technical Field
The invention relates to the field of oil and gas field exploration and development, in particular to a method for predicting the evolution rule of a rock mechanical layer of a fractured reservoir.
Background
With the discovery of more and more fractured reservoirs, the three-dimensional distribution of ground stress and natural fractures is gradually attracting attention of researchers nowadays, and the two factors are key factors for controlling the exploration and development of the fractured reservoirs. At present, a reservoir geomechanical method is a mainstream method for modeling ground stress and predicting structural cracks, and is effectively applied to a tight sandstone reservoir, a shale reservoir and a low-permeability sandstone reservoir, and accurate characterization of a rock mechanical layer is a key for effective application of reservoir geomechanical modeling, so that the accuracy of reservoir crack prediction and ground stress modeling is determined. The rock mechanical layer refers to a set of rock layers with consistent rock mechanical properties or similar rock mechanical behaviors, but the rock mechanical layer is not necessarily a lithologic uniform layer and does not completely correspond to a lithologic stratum. Fracture formations have also been used as synonyms for rock mechanics for a long time in the past, however, fracture formations reflect paleo-rock mechanics at the time of rock fracture, are affected by both diagenetic and structural effects, rock properties may change over time, rock mechanics controlling fracture development and rock mechanics suitable for predicting natural fractures may no longer exist.
The rock mechanical layer controls the development degree and the causative mechanism of natural cracks, and the development of the cracks also affects the magnitude and the anisotropism of rock mechanical parameters. The rock mechanical layer is subjected to double influences of diagenetic and constructional actions and is migrated, so that the rock mechanical layer is not required to be overlapped with lithologic stratum and fracture stratum, the rock mechanical layer for controlling the development of the fracture and the rock mechanical layer suitable for predicting the natural fracture distribution are not required to exist any more, and the research on the evolution rule of the rock mechanical layer is a new starting point of the fracture research of the next-generation reservoir. The invention discloses a method for predicting the evolution rule of a fractured reservoir rock mechanical layer under the control of a structural factor by adopting a reservoir geomechanical method.
Disclosure of Invention
The invention aims to solve the problems and provides a method for predicting the evolution law of a fractured reservoir rock mechanical layer, which realizes quantitative prediction of the evolution law of the fractured reservoir rock mechanical layer under the control of construction factors.
The technical scheme of the invention is as follows: the method for predicting the evolution rule of the rock mechanical layer of the fractured reservoir comprises the following specific steps (figure 1):
determining formation time of a structural crack by adopting a geological analysis and fluid geochemistry method; the geological analysis and fluid geochemistry method for determining formation period of the structural cracks refers to carrying out experimental study on fluid geochemistry evidence of reservoir crack activity based on crack multi-period filling characteristics on the basis of crack identification and characterization study, carrying out observation photographing, microscopic temperature measurement and laser Raman spectrum test on fluid inclusion in reservoir crack filling minerals, determining the type, morphology, phase state, abundance, salinity, composition and homogenization temperature, fe 2O3、MgO、MgO2, trace elements, carbon oxygen isotopes, formation water composition and mineralization degree of the fluid inclusion, calculating the capture pressure and density of the fluid inclusion, comparing and analyzing the difference of paleo-fluid properties of different types of fillers, combining the intersection relation of different groups of cracks of rock, comprehensively determining the period of reservoir crack activity, and setting the period of reservoir crack activity as N;
Secondly, selecting rock samples with undeveloped cracks to develop rock acoustic emission experiments, and determining the ancient and modern stress in the formation period of the structural cracks; the acoustic emission instrument is used for measuring the acoustic signals emitted by the inside of the rock in the rock loading process, and according to the Kaiser effect principle, when the stress of the rock reaches the maximum stress historically applied to the rock, the acoustic signals generated by the rock suddenly become larger, so that the ground stress value applied to the rock underground is determined.
And thirdly, on the basis of a uniaxial compression experiment, a triaxial compression experiment and well logging data calculation, determining the mechanical parameters of the complete rock with undeveloped cracks through uniaxial-triaxial and dynamic-static correction of the mechanical parameters of the rock, thereby providing a data basis for stress field numerical simulation and reservoir crack prediction.
The existing rock mechanical parameter measuring and calculating methods mainly comprise two types: firstly, rock mechanical experiments of rock samples are carried out in a laboratory, wherein the rock mechanical experiments comprise a single-axis compression experiment and a three-axis compression experiment, and the obtained results are commonly called static parameters; secondly, using geophysical data and combining a corresponding calculation model to calculate, wherein the obtained result is called a dynamic parameter; in addition, the rock mechanical parameters can be obtained by utilizing hydraulic fracturing data. In practical engineering applications, static rock mechanical parameters are typically employed. Among the static parameters, the triaxial compression experiment is more accurate than the uniaxial compression experiment in being closer to the actual environment of the underground rock, so that the triaxial compression experiment is a main basis for the ground stress and rock mechanical parameters used in the numerical simulation of the reservoir fracture.
The rock uniaxial and triaxial compression experiments directly utilize underground rock cores, belong to direct data, and have higher accuracy and credibility in theory. However, because of few sample points, the obtained result is directly used for numerical simulation, lacks sufficient theoretical basis, has higher experimental cost and is not economical. Well logging data make up for the defects of rock mechanics experiments to a certain extent, and have the advantages of good continuity, low cost and the like.
The rock mechanical parameters are explained by using logging data, and the related calculation formulas are as follows, wherein the data mainly comprise acoustic time difference, rock density, percentage of mud, rock porosity and the like:
Sc＝Ed[0.008Vsh+0.0045(1-Vsh)] (4)
In formulas (1) - (5): e d is dynamic Young's elastic modulus, MPa; mu d is dynamic poisson ratio, dimensionless; c is cohesive force and MPa; s c is compression strength, MPa; v sh is the percentage of argillaceous, and is dimensionless; ρ b is the rock density, kg/m 3;Δtp and Δt s are the longitudinal and transverse wave time differences, respectively, μs/ft; Is the internal friction angle, (°); phi is the logging porosity,%.
And taking rock mechanical parameters explained by logging data as constraints, inverting and determining the mechanical parameter three-dimensional heterogeneity of the complete rock with undeveloped cracks by adopting a well-seismic combination method or a phase attribute modeling method, and establishing a complete rock mechanical parameter three-dimensional heterogeneous model.
And fourthly, establishing a three-dimensional fracture discrete network model through field fracture observation, combining the size of the mechanical parameters of the complete rock, establishing a mathematical model among the normal stiffness coefficient, the shear stiffness coefficient and the normal stress of the fracture surface through fracture surface mechanical experiments, programming the mathematical model into a three-dimensional fracture discrete network numerical simulation program through computer programming, setting the corresponding fracture surface normal and shear stiffness values under different normal stress conditions by software for multiple times in each simulation, realizing the adoption of a custom fracture surface deformation constitutive model in the numerical simulation of the fractured rock mass, describing the deformation characteristics of the fracture surface, and finally establishing the three-dimensional fracture discrete network geomechanical model containing the fracture mechanical characteristics.
Fifthly, establishing a mathematical model of fracture parameters and rock equivalent mechanical parameters through discrete element numerical simulation, wherein the fracture parameters comprise fracture density, fracture azimuth and fracture included angle; the equivalent mechanical parameters are rock mechanical parameters which can generate the same deformation effect and the same breaking process as those of the fractured rock mass; and converting the discrete fracture network model into a continuous finite element model suitable for macroscopic stress field simulation through equivalent mechanical parameters.
And step six, based on the uniaxial-triaxial and dynamic-static correction of the rock mechanical parameters and the three-dimensional heterogeneous model of the complete rock mechanical parameters, establishing a three-dimensional model of a rock mechanical layer under the condition that cracks do not develop, and predicting the three-dimensional distribution of the 1 st stage ancient stress field and the structural cracks by adopting the three-dimensional model of the rock mechanical layer.
And seventh, on the basis of rock mechanical layer modeling, establishing a finite element model suitable for macroscopic stress field simulation, and taking the ancient stress size of the formation period of the structural crack as a constraint to obtain an ancient structural stress field suitable for predicting the 1 st period of the crack.
Eighth step, utilizing mathematical models between structural cracks and rock mechanical parameters, and predicting the density and the azimuth of the 1 st stage crack by combining the ancient structural stress field suitable for predicting the 1 st stage crack; in the three-dimensional stress field, predicting the occurrence of the crack according to a calculation model between the occurrence of the crack, the rock mechanical parameter and the stress field, wherein the calculation model between the occurrence of the crack, the rock mechanical parameter and the stress field is as follows:
In the numerical simulation of the stress field, the unit normal vector of the plane in which the crack is formed is assumed to be n ', the inclination angle is assumed to be η ', and the tendency is assumed to be γ '. According to the rock fracture criteria, the occurrence of cracks in a stress field coordinate system (the sigma 1、σ2、σ3 direction represents 3 coordinate axis directions) is obtained, and the included angles between the principal stress direction and the X-Y-Z axis in the earth coordinate system are respectively expressed as follows:
①.σ1 The angles with the X-Y-Z axes are expressed as: alpha 11、α12、α13;
②.σ2 The angles with the X-Y-Z axes are expressed as: alpha 21、α22、α23;
③.σ3 The angles with the X-Y-Z axes are expressed as: alpha 31、α32、α33.
Taking rock shear fracture as an example, the unit normal vector coordinates n "x、n″y、n″z of two sets of fracture faces generated in the stress field coordinate system are expressed as:
The vector n 'is represented in the geodetic coordinate system as 3 components n' x、n′y、n′z:
According to the above formula, the inclination angle η 'and the inclination γ' at the time of crack formation are calculated:
Obtaining the inclination angle eta' of the crack when forming:
the tendency gamma' of crack formation needs to be discussed in quadrants:
①n′x Gtoreq 0 and n' y >0, the tendency at crack formation is northeast, at which point:
②n′x Less than or equal to 0 and n' y >0, the tendency of crack formation is southeast, at this time:
③n′x <0 and n' y.ltoreq.0, the tendency at crack formation is southwest, at which point:
④n′x Gtoreq 0 and n' y <0, the propensity for crack formation is north west, at which time:
And in the three-dimensional stress field, determining the density distribution of the crack according to a calculation model between the crack density and the rock mechanical parameter and the stress field. The calculation model between the crack density and the rock mechanical parameter and stress field is as follows:
in the simulated stress field, if (σ 1+3σ3) >0, there are:
θ＝arccos[(σ1-σ3)/2(σ1+σ3)]/2 (14)
If (sigma 1+3σ3) is less than or equal to 0, θ=0, and the crack bulk density and the crack linear density are equal.
In the formula, omega f is the strain energy density required for newly increasing the surface area of the crack, J/m 3; omega is the total strain energy density of the rock, J/m 3;ωe is the elastic strain energy density that must be overcome to create a fracture, J/m 3; e is Young's modulus of elasticity, MPa; sigma 1、σ2、σ3 is the maximum, median and minimum effective principal stresses, MPa, respectively; σ p is the rock fracture stress, MPa; μ is the rock poisson ratio; e 0 is a scale factor related to lithology, dimensionless; d vf is fracture bulk density, m 2/m3; j is the energy required for generating a crack in unit area, J/m 2;Dlf is the crack linear density and bar/m; l 1、L3 is the length of the characterization unit body along the sigma 1、σ3 direction, and m; θ is the fracture angle of the rock, (°). The relevant mechanical parameters are determined by triaxial mechanical experiments of the rock.
And a ninth step of establishing a rock mechanical layer model suitable for predicting the next period of crack parameters according to a mathematical model between the construction cracks and the rock mechanical parameters and combining the 1 st period of crack density and azimuth, and circulating the seventh step and the eighth step to realize quantitative prediction of the N period of ancient stress field and the crack parameters of the corresponding period of time, and realizing prediction of the evolution rule of the crack reservoir rock mechanical layer according to the mathematical model between the construction cracks and the rock mechanical parameters.
The beneficial effects of the invention are as follows: and establishing a three-dimensional heterogeneous model of the mechanical parameters of the complete rock in the research area through core experiments, well logging calculation and seismic inversion. Establishing a three-dimensional discrete network model of the crack through field observation, adopting computer programming to program the mathematical model into a numerical simulation program of the three-dimensional discrete network of the crack, setting software to adjust the normal and shear stiffness values of the crack surface corresponding to different normal stress conditions for a plurality of times in each simulation, and establishing a geomechanical model of the three-dimensional discrete network of the crack, wherein the geomechanical model comprises mechanical characteristics of the crack. And analyzing the influence of the fracture parameters on the mechanical parameters and the anisotropism of the fractured rock mass through numerical simulation. And combining the relation between the fracture parameters of the reservoir and the rock mechanical parameters and stress fields, sequentially and circularly simulating the ancient stress fields in different periods and the densities and the shapes of the fractures in different periods, and finally interpreting the migration rule of the rock mechanical layer under the control of the construction factors. The invention provides a method for predicting the evolution law of a mechanical layer of a fractured reservoir rock from the angles of physical simulation and numerical simulation, which has higher practical value in the aspects of predicting the evolution of the mechanical property of the fractured reservoir rock, analyzing the mechanism of fracture origin, finely simulating an ancient stress field and the like, has low prediction cost and strong operability, and can greatly reduce the expenditure of manpower and financial resources.
Drawings
FIG. 1 is a flow chart of a method for predicting the evolution law of a fractured reservoir rock mechanical layer.
FIG. 2 (A) study area construction site; (B) eastern western section of the ados basin; (C) element 284 well length 6 reservoir group lithology synthetic histogram.
Fig. 3 shows the fracture strike versus principal stress azimuth in the source region of the erdosbasin (data Gao Shuai et al, 2015).
FIG. 4 (A) reservoir crustal stress testing apparatus; and (B) a rock acoustic emission experiment sampling schematic diagram.
FIG. 5 (A) element 284 pilot test area length 6 reservoir group rock dynamic-static Young's modulus relationship; (B) Element 284 pilot test zone length 6 reservoir group rock dynamic and static poisson ratio relationship.
Fig. 6 is a photograph of an extended set of field outcrop cracks for the lewisconsin basin west kerbstone ditch profile (view of the location of the field profile in fig. 1).
FIG. 7 is a diagram of (A) a three-dimensional crack network model of the Eartos basin west edge; and (B) a three-dimensional fracture discrete meta-model.
FIG. 8 shows the Young's modulus, poisson's ratio, E the Young's modulus, μ the Poisson's ratio of the rock at different orientations in a simulation unit of different dimensions and different positions; (data points of different colors represent rock mechanics parameter simulation results for different locations).
FIG. 9 is a graph showing the effect of the included angle between cracks on the rock mechanical parameters.
FIG. 10 is a graph showing the effect of Young's modulus of intact rock and fracture surface density on mechanical parameters of fractured rock mass in the horizontal direction; (A) horizontal maximum Young's modulus, (B) horizontal minimum Poisson's ratio, (C) horizontal average Young's modulus, (D) horizontal average Poisson's ratio, (E) horizontal minimum Young's modulus, and (F) horizontal maximum Poisson's ratio.
FIG. 11 (A) Yanshan geomechanical model; and (B) a Himalayan stage reservoir geomechanical model.
Fig. 12 is a prediction result of the evolution rule of the rock mechanical layer of the fractured reservoir. Yanshan period (A1-A3, B1-B3); himalayan stage (A4-A6, B4-B6); nowadays (A7 and B7); rock mass young's modulus distributions (A1, A4, and A7), poisson's ratio distribution in rock (B1, B4, and B7); a horizontal minimum principal stress distribution (A2, A5), a horizontal maximum principal stress distribution (B2, B5); crack density distribution (A3, A6), crack strike distribution (B3, B6).
Detailed Description
The following describes specific embodiments of the present invention with reference to the drawings:
The invention is described by taking the Western middle section element 284 of the slope of the northern Shanxi of the Huddos basin as an example. The Erdos basin is a middle-generation large-sized land basin overlapped on the Clatong plot of the ancient China north China, is a sedimentary basin (figure 2A) with earliest time and longest evolution time, is rich in oil and gas resources, and is a plurality of natural gas layers such as dwarf system, three-layer system and two-layer system of oil-containing layers of the midwife, two-layer system of the ancient China, carboy system, olympic system of the ancient China and the like. In addition, shale gas is enriched in the prolonged group of the midwife and the Shanxi group of the ancient world and the Benxi group. Wrinkles and faults in the erdos basin reservoir were relatively undeveloped (fig. 2B), but under regional structural stress, structural fractures of different dimensions developed extensively in the basin inner reservoir. The exploration and development practices show that natural cracks play a vital role in oil and gas resource exploration and development no matter a tight sandstone reservoir, a shale reservoir or a low-permeability sandstone reservoir, the cracks have obvious layer control characteristics, the occurrence is very stable, and the natural cracks are closely related to an ancient structural stress field in the formation period of the cracks; the direction of cracks in the Yanshan stage is mainly in the EW and SEE directions, and the direction of cracks in the Himalayan stage is mainly in the NS, NEE, NE directions. The method is affected by the loess topography of the earth surface, the basin seismic data has poor quality, the difficulty of crack prediction based on a seismic method is high, the reservoir geomechanical method is an effective method for constructing crack prediction, the method is continuously developed along with the natural crack distribution prediction of the reservoir, the theoretical model on which the natural crack distribution prediction method of the underground reservoir is built is also more and more emphasized, namely the precision of the geomechanical model directly determines the precision of the later stress field simulation and crack prediction. The study area Huaqing regional element 284 pilot test area, structurally located in the middle and south of the Erdos basin (FIG. 2A); the thickness of the layer of the target layer tri-stack extension group is 1000-1300 m, and the layer is in parallel non-integrated contact with the lower paper mill group and the upper dwarf Luo Tong Fu county group. Through years of oil gas exploration and development practice, the extension group is further divided into five lithology sections and 10 oil layer groups according to lithology, lake basin evolution history, logging small layer comparison and other data. From the typical well small layer division results, the whole upper parts of the long 6 1 layers, the long 6 2 layers and the long 6 3 1 layers are in a water-in deposition environment; the long 6 3 3 layers and the long 6 3 2 layers are integrally in a water-stripping deposition environment (figure 2C). The method for predicting the evolution rule of the mechanical layer of the fractured reservoir rock of the 284-block-shaped element in the western section of the slope of the north of the shan of the Erdos basin comprises the following steps:
The first step adopts geological analysis and fluid geochemistry to determine the formation period of the structural cracks, and as most of core cracks in a research area are unfilled cracks, the formation period of the cracks is mainly determined by referring to the research results of a horseland area adjacent to the research area and combining with the evolution of a regional stress field. According to the buried-thermal evolution history of the research area, the Yan mountain exercise IV curtain is prolonged to be between 85 and 116 ℃, and the time period of the Himalayan exercise I curtain is prolonged to be between 70 and 85 ℃. The uniform temperature range of quartz particle secondary salt water inclusion in the measured rock sample is 72-150 ℃, so that the inclusion with the estimated temperature range of 80-116 ℃ and 72-85 ℃ is respectively formed on a Yan motion IV screen and a Himalayan motion I screen, which indicates that the Yan motion IV screen-Himalayan motion I screen is the main formation period of the extended structure structural crack, wherein the number of inclusion in the temperature range of 85-116 ℃ is majority, the main development period of the crack is estimated to be the Yan motion IV screen, the effect of the structural activity intensity is achieved, and the density of the crack in the Yan period is Yu Xima Laiya period; the northeast and eastern cracks in the study area were mainly formed in the himalayan stage, the eastern and eastern cracks in the study area were formed in the Yanshan stage (fig. 3), and the crack formation stage in the study area was determined to be 2 stages, namely n=2.
Secondly, selecting a rock sample with undeveloped cracks to develop a rock acoustic emission experiment, and determining the ancient stress of the formation period of the structural cracks; the acoustic emission instrument is used for measuring the acoustic signals emitted by the inside of the rock in the rock loading process, and according to the Kaiser effect principle, when the stress of the rock reaches the maximum stress historically applied to the rock, the acoustic signals generated by the rock suddenly become larger, so that the stress applied to the rock in the underground, namely the ground stress value, is determined.
The acoustic emission experiment is completed by a gas reservoir ground stress test system (fig. 4A) of the institute of petroleum exploration and development in china, which is produced in the company GCTS in the united states, and can be loaded with 1500KN in the axial direction; the rigidity of the loading frame is 10MN; the pressure bearing of the pressure chamber is 140MPa and the temperature is 150 ℃; the confining pressure can be increased to 140MPa. As shown in fig. 4B, a single cylindrical small rock sample (Z axis) with a vertical direction Φ25×50mm was drilled on the full diameter core, and four cylindrical small rock samples of the same size were drilled in each plane perpendicular to the core axis at 45 ° intervals.
The rock acoustic emission experimental test results are shown in table 1, and the maximum level, the minimum level and the vertical main stress and the gradient of the rock sample are measured; the gradient of the horizontal maximum principal stress is about 0.020MPa/m, the gradient of the horizontal minimum principal stress is about 0.016MPa/m, the gradient of the vertical principal stress is about 0.025MPa/m, the horizontal principal stress gradient is basically consistent with the horizontal principal stress gradient of the south coal reservoir of the Hubei Doss basin, and the stress of the research area and the vicinity thereof is in a rule of linear increase along with the increase of depth. The rock acoustic emission determines that the current horizontal maximum main stress distribution range is between 41.3 and 45.3MPa, and the horizontal minimum main stress distribution range is between 33.3 and 36.7MPa; the change in vertical principal stress is mainly related to the density of the rock. Determining that the minimum level main stress in the Yanshan period is 41.29MPa and the maximum level main stress is 160.58MPa according to the distribution rule of acoustic emission ringing numbers; the level minimum main stress of the Himalayan stage is determined to be 33.18MPa, and the level maximum main stress is determined to be 108.54 MPa.
Table 1 rock acoustic emission experiment determines the magnitude of the principal stresses today
And thirdly, on the basis of a uniaxial compression experiment, a triaxial compression experiment and well logging data calculation, determining the mechanical parameters of the complete rock with undeveloped cracks through uniaxial-triaxial and dynamic-static correction of the mechanical parameters of the rock, so that a data basis is provided for the numerical simulation of the ancient stress field and the prediction of the cracks of the reservoir.
Rock mechanical parameters (mainly including rock poisson ratio, rock strength parameters, various elastic moduli, internal friction angles, cohesive forces and the like) are important basic data for performing ancient stress field simulation, current ground stress simulation, fracture dynamic and static parameter prediction, reservoir water injection pressure and other researches. Selecting a representative rock core, and processing the rock core into a rock sample with a flat end face, a diameter of 2.5cm and a length of 5.0cm by using equipment such as a drilling machine, a slicing machine and the like. The triaxial compressive strength experimental instrument adopts an MTS286 rock testing system of the middle petroleum exploration and development institute, and is tested according to the engineering rock mass testing method standard (GB/T50266-99). And establishing a mathematical model for converting the dynamic and static mechanical parameters of the rock, thereby laying a foundation for calculating the dynamic and static mechanical parameters of the rock for logging data. In the triaxial compression test process, a rock sample is placed in a high-pressure chamber, different confining pressures (0 MPa, 10MPa, 20MPa and 30 MPa) are applied to the periphery, the vertical stress of the rock is gradually increased, and the strain values of the rock sample in the axial direction and the radial direction are recorded respectively, so that a corresponding rock stress-strain curve is obtained. In this study, 9 representative rock samples were selected for triaxial compression experiments, and the results of some of the tests are shown in table 2.
Table 2 element 284 pilot test area long 6 oil layer group rock triaxial mechanical experiment data table
By calibrating the results of the dynamic mechanical parameters explained by rock mechanical experiments and well logging, a rock dynamic-static mechanical parameter conversion model is established to obtain static rock mechanical parameters (figure 5). The static mechanical parameters obtained by the formula in fig. 5 are not fully considered or hardly considered in practice, namely the obtained mechanical parameters can be regarded as mechanical parameter distribution when the natural fracture does not develop, so that the influence of the fracture on the macroscopic mechanical parameters of the rock mass and the size effect thereof need to be further analyzed to determine the current rock mechanical parameter distribution, and the current rock mechanical parameter distribution is further applied to the later stress field simulation and the current stress field numerical simulation.
And fourthly, establishing a three-dimensional fracture discrete network model through field fracture observation, and determining the mechanical parameter of the complete rock by combining the triaxial mechanical experiment of the rock. The mathematical model among the normal stiffness coefficient, the shear stiffness coefficient and the normal stress of the fracture surface is established through a fracture surface mechanical experiment, the mathematical model is programmed into a three-dimensional fracture discrete network numerical simulation program through computer programming, and the setting software adjusts corresponding fracture surface normal and shear stiffness values under different normal stress conditions for a plurality of times in each simulation, so that the deformation constitutive model of the fracture surface is adopted in the numerical simulation of the fractured rock mass, the deformation characteristics of the fracture surface are described, and finally the three-dimensional fracture discrete network geomechanical model containing the fracture mechanical characteristics is established.
The rock mechanics experiment is utilized to obtain a normal stress-normal displacement relation curve of the fracture surface, a mathematical model between the stress and the normal displacement of the fracture surface is established, a power function model is adopted to reflect the normal stress-normal displacement relation of the closed deformation of the fracture surface, and the relation between the normal stress (sigma n) and the normal displacement (S v) is expressed as follows:
σn＝1066.7Sv 1.4548 (17)
The relationship between fracture plane normal stiffness coefficient (K n) and normal stress (σ n) is expressed as:
Kn＝120.47σn 0.3126 (18)
The test result shows that the normal stiffness coefficient of the fracture surface increases along with the increase of the normal stress, and the two coefficients are in the same power law relation. The relation between the shear stiffness coefficient (K s) of the fracture surface and the normal stress (sigma n) is obtained by measuring the shear deformation of the fracture surface corresponding to different normal stresses:
Ks＝104.25σn 0.4812 (19)
the mathematical functions among the normal stiffness coefficient, the shear stiffness coefficient and the normal stress of the fracture surface are utilized, a Fish language is adopted to compile a mathematical model into a computer simulation program, and the setting software adjusts the corresponding mechanical parameters (normal and shear stiffness values) of the fracture surface under different normal stress conditions in 100 steps in each simulation, so that the deformation characteristics of the fracture surface are described by adopting a self-defined fracture surface deformation constitutive model in the numerical simulation of the fractured rock mass.
Through the field crack observation (figure 6) of the Erdossier extension group, a three-dimensional crack discrete network model (figure 7A) is established, a non-penetrating crack network model (figure 7B) is established in ANSYS software, the non-penetrating crack network model is imported into 3DEC software, and the size effect research of the mechanical parameters of the complex fractured reservoir is developed based on a three-dimensional discrete element method. And determining the mechanical parameters of the low-permeability sandstone reservoir by combining a triaxial mechanical experiment of the rock, wherein the initial Young modulus of the rock is set to 27GPa, the Poisson ratio is 0.25, and the density is 2.5g/cm 3 in numerical simulation.
And fifthly, establishing a mathematical model of fracture parameters and rock equivalent mechanical parameters through discrete element numerical simulation, wherein the fracture parameters comprise fracture density, fracture azimuth and fracture included angle. The equivalent mechanical parameters are rock mechanical parameters which can generate the same deformation effect and the same breaking process as those of the fractured rock mass; and converting the discrete fracture network model into a continuous finite element model suitable for macroscopic stress field simulation through equivalent mechanical parameters.
The reservoir mechanical parameters are different in different directions of the simulation unit under the influence of the cracks. Simulation calculation is carried out through a three-cycle method to obtain the change rules of mechanical parameters in different directions and different scales (figure 8); when the side length of the analog unit is small, it is difficult to accurately reflect the anisotropy of the mechanical parameters of the analog unit (fig. 8A to D). With further increase of the side length of the simulation unit (fig. 8E and F, r=1600 cm), the anisotropy of the mechanical parameters of the simulation unit is gradually clear, and the young modulus of the rock is relatively low in the direction of NE 40-50 ° and the direction of SEE115 °; in the NS direction and the EW direction, the Young's modulus of the rock is high; the variation rule of the Poisson's ratio is opposite to Young's modulus; however, in the same direction, the variation interval range of the Young modulus and the Poisson ratio of the rock is large, namely the mechanical parameters of the simulation units at different positions of the scale are obviously different from the actual mechanical parameters. When the side length of the simulation unit is further increased (fig. 8G and H, r=2400 cm), the anisotropy of the mechanical parameters of the simulation unit is further clear, and the mechanical parameters of the simulation units at different positions gradually tend to be consistent in different directions, namely, the simulated mechanical parameters are further approximate to real values; simulation results indicate that too small grid cells may not reflect the anisotropy of the rock mechanical parameters.
The extended set of eldos basin west edges cracks mainly develop two sets of structural cracks, and the production is stable, so that the influence of included angles among cracks, crack density and mechanical parameters of complete rock (mechanical parameters of complete rock) on equivalent mechanical parameters of fractured rock mass is mainly simulated. In order to systematically analyze the influence factors and evolution rules of the rock mechanical parameters, the Young modulus change rate and the Poisson ratio change rate are defined to describe the relative change of the rock mechanical parameters in different directions, namely the ratio of the equivalent Young modulus (Poisson ratio) of the fractured rock mass to the Young modulus (Poisson ratio) of the complete rock.
The anisotropy of rock mechanics depends on the included angle between cracks, and the influence of the included angle between cracks on the anisotropy of rock mechanics parameters is discussed by establishing a crack model of two groups of included angles. As shown in fig. 9A, the horizontal minimum young's modulus does not significantly change with increasing included angle between cracks during numerical simulation, the rate of change is about 0.975, the horizontal maximum young's modulus decreases with increasing included angle between cracks, and the vertical young's modulus decreases with increasing included angle between cracks. When the included angle between cracks=90°, the difference between the horizontal minimum young's modulus, the horizontal maximum young's modulus and the vertical young's modulus is minimum. As shown in fig. 9B, the rate of change of the horizontal minimum poisson's ratio with the included angle between cracks is about 1.025, and in the vertical direction, the poisson's ratio increases with the increase of the included angle between cracks, consistent with the law of change of young's modulus. Also, when the included angle between cracks=90°, the difference between the horizontal minimum poisson ratio, the horizontal maximum poisson ratio and the vertical poisson ratio is minimum.
By changing the fracture surface density of the simulation unit and the Young's modulus and Poisson's ratio of the complete rock, the influence of the complete rock mechanical parameters and the fracture surface density on the rock mass equivalent mechanical parameters is simulated, and as shown in FIGS. 10A and 10B, the influence of the fracture surface density on the horizontal maximum Young's modulus and the horizontal minimum Poisson's ratio is the smallest. The horizontal maximum Young's modulus value is slightly reduced with the increase of the fracture surface density, and the influence of the fracture surface density on the horizontal maximum Young's modulus is gradually reduced with the increase of the fracture surface density (> 1m/m 2); with increasing fracture surface density, the horizontal minimum poisson ratio gradually increases; when the fracture surface density is more than 1.5m/m 2, the fracture surface density has little effect on the horizontal minimum poisson's ratio. As shown in fig. 10C and 10D, the fracture surface density has an effect on the average young's modulus and the average poisson's ratio greater than the horizontal maximum young's modulus and the horizontal minimum poisson's ratio. The fracture surface density and the average Young's modulus are in a linear negative correlation and the average Poisson's ratio are in a linear positive correlation. As shown in fig. 10E and 10F, the fracture surface density has the greatest effect on the horizontal minimum young's modulus and the horizontal maximum poisson's ratio, and the fracture surface density has a linear negative correlation with the minimum young's modulus and a linear positive correlation with the maximum poisson's ratio. Simulation results show that the Poisson's ratio of the complete rock has smaller influence on the equivalent mechanical parameters of the simulation unit, the Young's modulus of the complete rock has more remarkable influence on the equivalent mechanical parameters of the simulation unit, and the larger the Young's modulus of the complete rock is, the larger the influence on the equivalent mechanical parameters of the fractured rock body is, namely the larger the Young's modulus of the complete rock is, the larger the equivalent Young's modulus of the simulated rock body is, and the larger the Poisson's ratio is increased.
And step six, based on the uniaxial-triaxial and dynamic-static correction of the rock mechanical parameters and the three-dimensional heterogeneous model of the complete rock mechanical parameters, establishing a three-dimensional model of the rock mechanical layer under the condition that cracks do not develop, and predicting the 1 st stage cracks by adopting the three-dimensional model of the rock mechanical layer.
And a seventh step of establishing a finite element model suitable for macroscopic stress field simulation on the basis of rock mechanical layer modeling, wherein the total thickness of the Yanshan-stage geomechanical model is 4500m, the burial depth of a sand body with a target layer length of 6 3 is 1990m, the horizontal minimum main stress is 41.29MPa, the horizontal maximum main stress is 160.58MPa, the ancient stress in the formation period of the structural crack is taken as a constraint, and the ancient structural stress field suitable for predicting the 1 st stage crack is predicted.
The eighth step is that by analyzing the simulation results of the stress field in the research area Yan Shanqi and the himalayan stage, the vertical layering phenomenon of the stress field is particularly obvious, which is probably the main reason for the vertical layering of the construction cracks nowadays, and the distribution of the horizontal minimum main stress and the horizontal maximum main stress are closely related to the sand body distribution. Using the established three-dimensional Yanshan stage complete rock mechanical parameter distribution model (figures 12A1 and 12B 1), establishing a corresponding reservoir geomechanical heterogeneous model, and simulating to obtain Yanshan stage stress field distribution (figures 12A2 and 12B 2); predicting the density and orientation of the 1 st stage fracture by using a mathematical model between the structural fracture and the rock mechanical parameters and stress fields (figures 12A3 and 12B 3);
And a ninth step of establishing a rock mechanical layer model suitable for predicting the crack parameters of the next stage according to a mathematical model between the structural crack and the rock mechanical parameters and combining the crack density and the azimuth of the 1 st stage, wherein when the total thickness of the geomechanical model of the Himalayan stage is 4500m, the horizontal minimum principal stress is 33.18MPa, and the horizontal maximum principal stress is 108.54MPa. By analysis of reservoir burial history, a 63 sand burial depth of 2150m was obtained (fig. 11B). And (3) circulating the seventh step and the eighth step, realizing quantitative prediction of the parameters of the fracture in the ancient stress field of different periods, and realizing prediction of the evolution rule of the mechanical layer of the fractured reservoir rock.
And (3) obtaining rock equivalent mechanical parameters after development of the cracks in the Yanshan stage by using a mathematical model between the rock equivalent mechanical parameters and the crack parameters, establishing a geomechanical heterogeneous model (fig. 12A4 and 12B 4) of the reservoir in the Himalayan stage, simulating to obtain stress fields (fig. 12A5 and 12B 5) in the Himalayan stage, and further determining the surface density and the occurrence of the cracks in the Himalayan stage (fig. 12A6 and 12B 6). And combining Yan Shanqi and fracture parameters (shape, density and combination mode) in the Himalayan period to obtain the current fracture reservoir rock mechanical parameter distribution (figures 12A7 and 12B 7), and establishing a reservoir geomechanical heterogeneous model to simulate and obtain the current ground stress field.
Comparing the rock mechanical parameter distribution (fig. 12A1, 12A4, 12A7, 12B1, 12B4 and 12B 7) in different periods can be obtained, the rock equivalent young modulus generally tends to decrease from the Yanshan period to the himalayan period to the present day, and the equivalent poisson ratio generally increases. The difference of the equivalent Young modulus and the Poisson ratio of the rock mass in the first 284 pilot test area generally shows a reduced trend, namely the Young modulus is larger in the Yan mountain period, and the Young modulus is greatly reduced in the Himalayan period and the current period; in contrast, the young's modulus in the himalayan period was small in the well region with smaller young's modulus in the himalayan period and the young's modulus in the present day was reduced in magnitude, even without change (crack does not develop) (fig. 12A1, 12A4, and 12 A7). Similarly, in the well region where the Poisson's ratio is small in the Yan-mountain period, the Poisson's ratio increases with today in the Himalayan period, whereas in the Poisson's ratio is large, the Poisson's ratio increases with today in the Himalayan period are small, and even do not change (crack does not develop) (FIGS. 12B1, 12B4, and 12B 7).
The present invention has been described above by way of example, but the present invention is not limited to the above-described embodiments, and any modifications or variations based on the present invention fall within the scope of the present invention.
Claims (4)
1. The method for predicting the evolution rule of the rock mechanical layer of the fractured reservoir comprises the following implementation steps:
Determining formation periods of structural cracks by adopting a geological analysis and fluid geochemistry method, and setting the period of reservoir crack activity as N;
Secondly, selecting rock samples with undeveloped cracks to develop rock acoustic emission experiments, and determining the ancient and modern stress in the formation period of the structural cracks; the acoustic emission instrument is used for measuring acoustic signals emitted by the inside of the rock in the rock loading process, and according to the Kaiser effect principle, when the stress of the rock reaches the maximum stress historically applied to the rock, the acoustic signals generated by the rock suddenly become larger, so that the ground stress value applied to the rock in the underground is determined;
Thirdly, on the basis of a uniaxial compression experiment, a triaxial compression experiment and well logging data calculation, determining the mechanical parameters of the complete rock with undeveloped cracks through uniaxial-triaxial and dynamic-static correction of the mechanical parameters of the rock; taking rock mechanical parameters interpreted by logging data as constraints, inverting and determining three-dimensional heterogeneity of complete rock mechanical parameters of which cracks do not develop by adopting a well-seismic combination method or a phase attribute modeling method, and establishing a complete rock mechanical parameter three-dimensional heterogeneous model;
Establishing a three-dimensional fracture discrete network model through field fracture observation, and establishing a mathematical model among a fracture surface normal stiffness coefficient, a shear stiffness coefficient and normal stress through a fracture surface mechanical experiment by combining the mechanical parameters of the complete rock; the mathematical model is programmed into a three-dimensional fracture discrete network numerical simulation program through computer programming, and software is set to adjust the normal direction and shear stiffness values of the corresponding fracture surface under different normal stress conditions for a plurality of times in each simulation, so that the fact that in the numerical simulation of a fractured rock mass, a self-defined fracture surface deformation constitutive model is adopted, deformation characteristics of the fracture surface are described, and a three-dimensional fracture discrete network geomechanical model containing fracture mechanical characteristics is established;
Fifthly, establishing a mathematical model of fracture parameters and rock equivalent mechanical parameters through discrete element numerical simulation, wherein the fracture parameters comprise fracture density, fracture azimuth and fracture included angle; the rock mass equivalent mechanical parameters are rock mechanical parameters which can generate the same deformation effect and the same breaking process as those of the fractured rock mass; converting the discrete fracture network model into a continuous finite element model suitable for macroscopic stress field simulation through equivalent mechanical parameters;
step six, based on uniaxial-triaxial and dynamic-static correction of rock mechanical parameters and a complete rock mechanical parameter three-dimensional heterogeneous model, establishing a rock mechanical layer three-dimensional model under the condition that cracks do not develop, and predicting the 1 st stage ancient stress field and constructing crack three-dimensional distribution by adopting the rock mechanical layer three-dimensional model;
A seventh step of establishing a finite element model suitable for macroscopic stress field simulation on the basis of rock mechanical layer modeling, wherein the ancient stress in the formation period of the structural crack is taken as a constraint, and an ancient structural stress field suitable for predicting the 1 st period of the crack is obtained;
Eighth step, utilizing mathematical models between structural cracks and rock mechanical parameters, and predicting the density and the azimuth of the 1 st stage crack by combining the ancient structural stress field suitable for predicting the 1 st stage crack; predicting the occurrence of the crack in the three-dimensional stress field according to the occurrence of the crack and the calculation model between the rock mechanical parameters and the stress field; predicting the three-dimensional distribution of the crack density according to a calculation model between the crack density and the rock mechanical parameter and stress field;
And a ninth step of establishing a rock mechanical layer model suitable for predicting the next period of crack parameters according to a mathematical model between the construction cracks and the rock mechanical parameters and combining the 1 st period of crack density and azimuth, and circulating the seventh step and the eighth step to realize quantitative prediction of the N period of ancient stress field and the crack parameters of the corresponding period of time, and realizing prediction of the evolution rule of the crack reservoir rock mechanical layer according to the mathematical model between the construction cracks and the rock mechanical parameters.
2. The method for predicting the evolution law of a fractured reservoir rock mechanical layer according to claim 1, wherein the method comprises the following steps:
The geological analysis and fluid geochemistry method for determining formation period of the structural cracks refers to carrying out experimental study on fluid geochemistry evidence of reservoir crack activity based on crack multi-period filling characteristics on the basis of crack identification and characterization study of different scales, carrying out observation photographing, microscopic temperature measurement and laser Raman spectrum test on fluid inclusion in reservoir crack filling minerals, determining the type, morphology, phase state, abundance, salinity, composition and homogenization temperature, fe 2O3、MgO、MgO2, trace elements, carbon oxygen isotopes, formation water composition and mineralization degree of the fluid inclusion, calculating the capture pressure and density of the fluid inclusion, comparing and analyzing the difference of paleo-fluid properties of different types of fillers, combining the intersection relation of different groups of cracks of rock, and comprehensively determining the period of reservoir crack activity.
3. The method for predicting the evolution law of a fractured reservoir rock mechanical layer according to claim 1, wherein the method comprises the following steps:
The calculation model between the fracture occurrence and the rock mechanical parameter and stress field is as follows:
In the numerical simulation of the stress field, the unit normal vector of the plane in which the crack is formed is set as n ', the inclination angle is set as eta ', and the tendency is set as gamma '; according to the rock fracture criteria, the occurrence of cracks in the stress field coordinate system is obtained, and the included angles between the main stress direction and the X-Y-Z axis in the geodetic coordinate system are respectively expressed as follows:
①.σ1 The angles with the X-Y-Z axes are expressed as: alpha 11、α12、α13;
②.σ2 The angles with the X-Y-Z axes are expressed as: alpha 21、α22、α23;
③.σ3 The angles with the X-Y-Z axes are expressed as: alpha 31、α32、α33;
Taking rock shear fracture as an example, the unit normal vector coordinates n "x、n″y、n″z of two sets of fracture faces generated in the stress field coordinate system are expressed as:
The vector n 'is represented in the geodetic coordinate system as 3 components n' x、n′y、n′z:
According to the above formula, the inclination angle η 'and the inclination γ' at the time of crack formation are calculated:
Obtaining the inclination angle eta' of the crack when forming:
the tendency gamma' of crack formation needs to be discussed in quadrants:
①n′x Gtoreq 0 and n' y >0, the tendency at crack formation is northeast, at which point:
②n′x Less than or equal to 0 and n' y >0, the tendency of crack formation is southeast, at this time:
③n′x <0 and n' y.ltoreq.0, the tendency at crack formation is southwest, at which point:
④n′x Gtoreq 0 and n' y <0, the propensity for crack formation is north west, at which time:
4. the method for predicting the evolution law of a fractured reservoir rock mechanical layer according to claim 1, wherein the method comprises the following steps: the calculation model between the crack density and the rock mechanical parameter and stress field is as follows;
in the simulated stress field, if (σ 1+3σ3) >0, there are:
θ＝arccos[(σ1-σ3)/2(σ1+σ3)]/2(14)
If (sigma 1+3σ3) is less than or equal to 0, the θ=0, and the crack bulk density is equal to the crack linear density;
In the formula, omega f is the strain energy density required for newly increasing the surface area of the crack, J/m 3; omega is the total strain energy density of the rock, J/m 3;
Omega e is the elastic strain energy density that must be overcome to create a fracture, J/m 3; e is Young's modulus of elasticity, MPa; sigma 1、σ2、σ3 is the maximum, median and minimum effective principal stresses, MPa, respectively; σ p is the rock fracture stress, MPa; μ is the rock poisson ratio; e 0 is a scale factor related to lithology, dimensionless; d vf is fracture bulk density, m 2/m3; j is the energy required for generating a crack in unit area, J/m 2;Dlf is the crack linear density and bar/m; l 1、L3 is the length of the characterization unit body along the sigma 1、σ3 direction, and m; θ is the fracture angle, degree of the rock; the relevant mechanical parameters are determined by triaxial mechanical experiments of the rock.
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